In August of 2020, an unusual geologic phenomena began in the northern Antarctic.
Near the South Shetland Islands, a highly energetic swarm of earthquakes occurred.
Although this region often is the site of moderate magnitude earthquakes, this series
was different due to the sheer number and magnitude of quakes which occurred.
Over the next 6 months, earthquakes continued to get shallower and shallower.
Then, earthquake activity declined until it returned to normal levels.
In total, 85,000 earthquakes occurred, the largest of which was a magnitude 6.0.
Later analysis revealed that these quakes were not tectonic in origin, but instead occurred
due to the movement of a massive intrusion of magma deep in the crust.
Estimates suggest that an unusually large amount of magma was involved, totalling 0.41
cubic kilometers of molten rock.
For comparison, that is 20 times larger than the ongoing intrusion at Sao Jorge in the
Azores, or about 3 times larger than the total volume of lava erupted during the 6 month
eruption of Geldingadalir in Iceland.
Even more unusual was that this intrusion was centered to the east of a submarine volcano
which was considered to be either extinct or dormant.
The volcano I am referring to is the Orca Seamount.
The Orca Seamount volcano is located south of the South American continent where it is
930 kilometers south-southeast of the southernmost point of mainland Argentina.
This volcano is a large shield volcano composed out of primarily basalt and basaltic andesite
lavas.
Its summit is located approximately 600 meters below sea level, and its edifice measures
11 kilometer wide.
The most prominent feature of the Orca Seamount is its central caldera which measures 3 kilometer
wide and 500 meters deep.
The reason why this seamount exists is complex.
Around 4 million years ago, the Phoenix tectonic Plate was subducting underneath the Antarctic
plate at a very shallow angle.
After several hundred thousand years of subduction, something changed.
The Phoenix tectonic plate suddenly snapped downwards at a much steeper angle in a process
called slab rollback.
This caused magma to rush upwards to fill the empty space previously occupied by the
tectonic plate.
As large volumes of magma intruded into the crust, the crust began to thin.
This caused the crust to begin spreading apart to the north and south, resulting in the Bransfield
Basin.
It is within this rift zone that the Orca Seamount formed, along with the adjacent Deception
Island caldera and Penguin Island volcano.
The earthquake swarm began in August of 2020, when a magma chamber at a depth of 20 kilometers
produced a large batch of molten rock which slowly headed towards the crust mantle boundary.
This boundary at 15 kilometers depth was reached 2 weeks later in late August, when earthquake
activity intensified.
From late August to mid October, magma continued rising shallower into the crust, generating
numerous earthquakes through two separate lobes of material and eventually reached a
depth of merely 5 kilometers.
Afterwards, upward mobility of the material seemingly stopped and instead it spread out
in a primarily horizontal orientation, forming a 20 kilometer long dike of magma.
This dike was elongated in a northeast orientation and produced increasingly frequent quakes.
Then, on November 6th, a magnitude 6.0 earthquake occurred.
Suddenly, the dike intrusion of magma stopped expanding, and earthquake activity quickly
declined until it ceased in February of 2021.
It is suspected although not proven that the decline in earthquake activity was caused
by pressure release related to a yet to be discovered large volume lava flow related
to a submarine volcanic eruption.
The discoveries of both this recent magma intrusion in Antarctica along with a newly
built submarine shield volcano near Mayotte are a reminder that little is known about
the vast number of submarine volcanoes as less than 20% of the ocean floor has ever
been fully mapped or explored.
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